The present invention comprises a pultruded product which contains, in addition to the usual glass roving strands, a cellulosic mat which serves as a filler or reinforcing filler. The invention further comprises a method for making products of the above type.
Pultrusion is a process for continuously forming reinforced plastic materials having a uniform cross-sectional profile. The word "pultrusion" is a hybrid which combines the words "pull" and "extrusion". The product is literally pulled through a forming die. In its most usual form, pultrusion involves feeding a multiplicity of fiberglass roving strands, with or without additional plies of glass mat of appropriate width, into a pultrusion die. A resin, normally a thermosetting material such as a polyester, is injected into the die where it is uniformly distributed among the reinforcing materials. Alternatively, the reinforcing material may be drawn through a resin bath prior to entry into the die. The die itself is heated. As the product is drawn from the die, the resin is either cured, or very nearly cured. The endless product so formed is then cut to appropriate length. Many variations of this general process have been developed as the technology has matured.
Pultruded products are used in a great variety of applications. In many places they have replaced metallic construction materials, particularly those used in highly corrosive environments. Structural beams, floor gratings, handrails, ladders, and many similar products are now made by a pultrusion process.
A general background on pultrusion is found in an article by Martin, Modern Plastics Encyclopedia, pp. 317-318, McGraw-Hill, Inc., New York (1986). Another good background discussion is found in U.S. Pat. No. 4,252,696 issued to McQuarrie.
Despite the enormous versatility of the pultrusion method and the many variations which have been developed around it, it still has limitations. In many cases it is necessary to overdesign products in order to ensure uniform distribution of the glass reinforcement within the resin matrix. Products with low glass content tend to show areas of resin separation in which the reinforcing material may be completely absent. This resin separation is apt to occur even when high concentrations of mineral fillers are used with the resin. Pultruded products also tend to have relatively high density in comparsion to many other plastic composites. This results in a relatively high cost per unit volume of the finished product. Further, because of the problem of resin separation at low reinforcing fiber contents, it is very difficult to make products less than about three millimeters in thickness, even though they may not be required to have high strengths. While some attempts have been made in the prior art to address these deficiencies, none have been particularly successful to the present time.
Reference might be made at this point to a number of patents of general pertinence to the present invention. Cogswell et al, U.S. Pat. No. 4,541,884, describe pulling a continuous tow or roving of fibers through a mixture of a thermoplastic polymer and a volatile plasticizer. The presence of the plasticizer reduces the melt viscosity to the point that uniform impregnation of the reinforcing fibers is possible. After forming the product, the plasticizer is volatilized. The invention suggest that glass fiber, carbon fiber, jute and high modulus synthetic polymer fibers can be used for reinforcing. However, the reinforcing must have sufficient longitudinal strength to enable it to be drawn through the viscous impregnation bath. The inventors further note that at least 50% by volume of the fibers must be aligned in the direction of draw.
Goppel et al, U.S. Pat. No. 4,028,477, make a pultruded product by first taking an open cell foamed core material and impregnating it with a thermosetting resin. The impregnated foam core is faced on one or both sides with a resin free fibrous reinforcing layer. The assembly is then molded in a pultrusion die where the resin flows from the foam into the reinforcement. The foam core is ultimately totally collapsed in the process. Cellulosic paper, cotton fabric, asbestos, nylon, and glass are disclosed as reinforcing materials.
Tadewald, in U.S. Pat. No. 4,207,129, describes a pultruded product which is a conductive or semi-conductive sheet. As a first step in its manufacture, a resin is admixed with a semiconductive powder, such as acetylene black, and a refractory inorganic oxide. This mixture is impregnated into a supporting material which may include a heavy absorptive paper about 0.020 inches in thickness. The resin in the impregnated product is then B-staged. At this time conductive elements such as copper foil strips may be included. The B-stage material is then encapsulated with glass rovings and/or scrim and further impregnated with a polyester resin in a conventional pultrusion process.
Cellulosic materials have found very little use in any capacity in reinforced plastic materials based on polyester resins. They have had a long standing reputation, not without some justification, for causing soft cures and tacky surfaces. This has been particularly true for products based on wood fiber which have not been chemically modified. Gregory et al, in U.S. Pat. No. 3,361,690, describe the use of Douglas-fir bark fiber as a reinforcing material for polyester-based bulk molding compounds. Gehr, in U.S. Pat. No. 3,248,467, describes the use of similar compounds in melamine overlaid reinforced plastic moldings. However, the bark fiber products appear to be an exception to the problems encountered with other cellulose based materials.
Purified cellulose has found widespread use in thermosetting resins such as impregnated phenolic and melamine laminates and molding conpounds. However, it has not been generally regarded as useful in pultruded products. A few applications using helical wound paper have appeared in the paten literature. Japanese patent application No. 56-17245 describes the use of a low density (ca. 0.7 g/cc or lower) paper tube which serves as a permanent mandrel for a pultruded cylindrical shape. The resin and glass composite surrounding the tube is bonded only to the surface and the tube itself is not impregnated with resin.
French patent application No. 2,391,067 teaches the use of a plurality of reinforcing fiber bundles, each of which is wrapped with a barrier material that can be paper. These may then be used in pultruded or extruded products. The barrier material serves to prevent passage of resin into the reinforcing fiber bundles. The application notes that it is important that the resin not impregnate the barrier layer to any extent.
Meyer, In U.S. Pat. No. 3,470,051, describes a relatively complex helically wound product that is preformed and then used as a core for pultruded products such as arrow shafts. The preform has a double layer of paper, then a layer of glass, and finally another layer of paper. This is then coated with resin and molded into a rod before use in the pultrusion process.
An early article describing the pultrusion process (Machine Design, 43, Dec. 26, 1971, pp. 45-49) speculates that any material that can be fed from a coil is a "possibility" for use in the pultrusion process. Paper products, along with a host of other materials, are suggested as being potentially useful.
In none of these cases is a cellulose-based material completely resin saturated and copultruded with a reinforcing material such as glass rovings. Surprisingly, the present inventor has found that cellulose-based mats can be successfully used in pultruded products made with a wide variety of impregnating resins. Apparently the present success has been due in great part to the fact that it has been found critical to maintain a moisture content no greater than about 3% in the cellulosic product if voids and blistering are to be avoided.